1. Field of the Invention
This invention relates to an ophthalmological taking instrument in which a barrier filter is inserted into a photographing optical path at the time when a fluorescence photography is taken.
2. Prior Art of the Invention
Heretofore, when an eye fundus is taken by fluorescence photography, a fluorescent agent is injected to the vein of the patient. The fluorescence photography is performed in such a manner as that when a fluorescent agent circulates within the body of the patient and the fluorescent agent appears on the eye fundus, the eye fundus is illuminated with an exciting light of a predetermined wavelength so that fluorescence of a predetermined wavelength is emitted from the fluorescent agent on the eye fundus. At the time when the eye fundus is taken by fluorescence photography, an exciter filter for permitting an exciting light of a predetermined wavelength to transmit therethrough is inserted into an illuminating optical path and a barrier filter for permitting fluorescence of a certain wavelength to transmit therethrough and removing the exciting light of a predetermined wavelength is inserted into the photographing optical path.
The quantity of the fluorescent agent flowing in the blood vessel of the eye fundus, as shown in FIG. 6, is gradually increased in initial period T after the passage of a predetermined time from the time when the fluorescent agent is injected. The agent is abruptly increased in quantity in a middle period T2 and begins to appear on the entire eye fundus. In a latter time period T3, the quantity of the fluorescent agent is considerably reduced and further gradually reduced with the passage of time.
As a consequence, in the latter time period T3, because brightness of the fluorescence is low and the barrier filter is inserted in the photographing optical path, the whole eye fundus is observed dim. Therefore, as the alignment becomes difficult to perform, it is necessary for the examiner to increase the quantity of the illumination light.
However, when taking into consideration possible damage to the retina of the eye, it is undesirable to increase the quantity of the illumination light more than necessary. Therefore, there is such a problem as that alignment is difficult to perform in the later time period T3.